Vulcanized fluoroelastomers have been used in a variety of applications, in particular for manufacturing sealing articles such as oil seals, gaskets, shaft seals and O-rings, because of several desirable properties such as heat resistance, chemical resistance, weatherability, etc.
It is nevertheless required for “as polymerized” fluoroelastomers to undergo curing/crosslinking processes (so-called “vulcanization”) in order to ensure required sealing and mechanical properties to be exhibited in final parts.
Several techniques have been developed for ensuring creation of a three-dimensional cured structure able to deliver expected performances; the underlying chemistry generally requires a crosslinking agent to provide for connections between fluoroelastomer polymer chains. Nature and reactivity of the crosslinking agents used enable categorizing cross-linking systems. Most used systems are based either on polyhydroxyaromatic compounds (typically bi-phenols), reacting through “ionic” chemistry in the presence of basic compounds and onium accelerators, via displacement of acidic hydrogen atoms in the chain, or on poly-unsatured compounds, reacting via radical chemistry in the presence of peroxide(s), via displacement of labile groups (typically iodine or bromine atoms) in the chain.
Ionic cross-linking generally leads to cured parts which, although possessing appreciable thermal resistance, yet may be inadequate in certain thermal demanding conditions; furthermore, ionically cured parts are somehow sensitive to certain chemicals, in particular certain acids and bases. Their processing is delicate because of fouling phenomena which may arise, and incorporation of curing agents and accelerant can be a delicate task for ensuring homogenous dispersion and hence homogeneous crosslinking network.
Similarly, peroxide curing lead to parts which can be susceptible of chemical attacks, and may suffer of inadequate thermal resistance in highly demanding applications; on top of this, sealing performances obtained in parts cured by peroxide curing can be poorer than those of similar base elastomers cured by ionic route.
Now, while it is acknowledged that overall performances of the fluoroelastomers will depend upon the nature, molecular structure and composition of the fluoroelastomer itself, it is nevertheless true that chemistry of cross-linking may have a significant impact on the same. Indeed, because cross-linking agents are generally hydrogen-containing compounds, possessing possibly labile and/or reactive groups, and optionally necessitating, for their effectiveness, addition of modifying compounds which may detrimentally affect thermal and/or oxidative stability of the fluoroelastomer, there is a continuous quest in the art for alternative and improved crosslinking system, based on alternative chemistry, delivering outstanding crosslinking ability and hence sealing performances in cured parts, being hence safer and cheaper than the above, and which could obviate to the drawbacks of the ionic or peroxide cross-linking systems of the prior art.